1st annual offline tournament "Best Of The Best"

I reached the sea at the beginning of the 3rd week, then captured 2 Castles and fought against Arham. And I cleared the swamp with my second hero (in my case, it was a Celt) a little later. In general, I could have completed it in 15-20 days, but I spent too much time on unnecessary things (for example, the Celt only got navigation in the Witch's Hut on the 3rd week => he spent a long time sailing to the orange town, and so on), and I didn't have enough time or energy to replay it one more time.
I think the winners used the strategy of having the main hero run to a town and then transferring his army to him in a chain. I just don't understand how they dealt with Magnus so quickly, in terms of how that battle went.